(1) Field of the Invention
The invention relates to modems for use in computers. More particularly it relates to a method and apparatus for minimizing the power consumption of a modem while maintaining maximum data throughput.
(2) Description of the Prior Art
Modems are data communications devices that provide connections for computers into the public switched telephone network. A sending modem converts digital signals generated by a host computer to analog signals for transmission over telephone lines. A receiving modem then reconverts the analog signals back to digital signals which are then passed to the receiving modem's host.
In battery powered computers such as lap top PC's, battery life is a critical concern to users and therefore to system designers. In addition so called "green" desk top computers are now being designed that place a high priority on minimizing power consumption. In the past, computers and particularly laptop computers have been designed to automatically power down into a low power state after a certain period of inactivity. If the laptop was using a modem at the time of automatic powerdown, the computer automatically hung up the modem. That is, the connection through the telephone system was dropped.
While this is a good power saving technique, it makes for slow data transfer in the cases where the user is interested in reconnecting to the same remote modem. This is because of the long time required to make a modem connection. First, the number of the destination modem must be dialed. Next, the destination modem rings and after several rings, the line is picked up. Next, a signal exchange called a "handshake" takes place that establishes the parameters of a communication. This process takes considerable time. In addition, modern high speed modems have distortion compensation features that make this process even more time consuming. All transmitted signals are susceptible to distortion. For example, some distortion is introduced by the medium itself such as signal attenuation. Some distortion is due to outside interference such as background noise, storms, emf from machinery, multi path transmission interference, etc. All of these factors result in nonuniformities in the amplitude and phase characteristics of the transmission channel. With data transmission, distortion is more of a problem than with voice since the human brain can deal with a considerable degree of distortion and still capture the message. Not so with computers and data transmission. Each time a new call is made, the distortions are different than in the previous calls. The problem is exacerbated if one or both of the communication points is mobile since this causes the distortion to change--sometimes radically--in very short periods of time. Modern modems have dealt with this problem by adding adaptive distortion compensation. That is, logic and circuitry are added to the modem that compensates for less than ideal amplitude and phase linearity in the received signal. Furthermore, the compensation is adaptive in that it adjusts itself to changes in the distortions as they are measured at the receiving station. But, each time the telephone line is dropped, the training of the adaptive circuits must start over. This also takes considerable time. Moreover, it may not be possible to reconnect at all because the line is now busy. Thus, the strategy of disconnecting has a high penalty of reduced throughput.
Thus, current modems do a poor job of both maintaining data throughput and maximizing battery life.